Method for lactic acid oligomerization

ABSTRACT

The invention concerns a method for continuously producing a dimmer-rich and tri-mer rich oligomeric lactic acid composition, starting with a lactic composition by eliminating water from said lactic acid composition. The invention is characterized in that it consists in: carrying out said elimination in an adiabatic reactor provided with a recycling loop, continuously introducing a lactic acid composition in a circulating starter reaction medium being brought to a temperature ranging between 120° C. and 180° C. and in eliminating the water vapour form.

This application claims benefit, under U.S.C. §119 or §365 of FrenchApplication No. 02/01549, filed Feb. 8, 2002; and PCT/FR03/00305 filedJan. 31, 2003.

FIELD OF THE INVENTION

The present invention relates to a method for obtaining an oligomericlactic acid composition, said composition being suitable for preparingin particular an oligomeric composition of esters of lactic acid or forobtaining lactide which is the precursor of choice for obtainingpolylactides used as biomaterials.

BACKGROUND OF THE INVENTION

In the international patent application WO 01/47860-A1, a method isproposed for converting a lactic acid composition to a lactic acid estercomposition, which consists, in a first step, of the oligomerization ofa lactic acid composition.

Lactic acid composition means in particular any aqueous solution oflactic acid, regardless of its method of preparation and its properties,said solution, for example, presenting a highly variable dry mattercontent (DM) and lactic acid purity. It may relate in particular totrade solutions containing 50, 80, 88 or 90% of dry matter, with theunderstanding that such solutions are in fact mixtures of water,monomers, dimers and higher oligomers of lactic acid.

In said application WO 01/47860, the conversion of the lactic acidcomposition to an oligomeric lactic acid composition takes place by theremoval of water which can advantageously be achieved by simpleevaporation, in one or a plurality of steps, at a temperature of 100° C.to 170° C., at atmospheric pressure or under reduced pressure, until anoligomeric composition is obtained having a mean degree ofpolymerization (MDP) between 2 and about 30 and preferably between 3 and10.

According to said application WO 01/47860, the preparation of anoligomeric composition having such an MDP offers the advantage ofproviding (of having) a limited water content and consequentlyfacilitates its subsequent esterification.

The MDP is calculated by the following formula:MDP=18/[100×90.08]/T−72   (1)where T is the mass of lactic acid monomer (CH₃—CHOHCOOH, 90.08 g/mol)contained in 100 g of oligomeric lactic acid composition. This mass isdetermined after saponification (20 min at 100° C.) by excess causticsoda, of a given weight of sample (0.1 g to 0.3 g) of the oligomericcomposition. After neutralization of the reaction mixture, the latter israpidly analyzed by high performance liquid chromatography (HPLC) withrefractometric detection. This analysis can be performed on a “SHODEX SH1011” type cation exchange column using N/100 sulfuric acid as eluent.This technique serves to determine the mass of lactic acid monomercontained in the sample tested and, by simple calculation, the mass Tcontained in 100 g of oligomeric composition.

By way of example, an oligomeric lactic acid composition is obtained inapplication WO 01/47860 having an MDP of 5.9, which, if the precedingformula (1) is applied, corresponds to a mass of lactic acid T of119.92.

The lactic acid oligomers and their distribution in the oligomericlactic acid composition can be determined by gas chromatography GC on aShodex KF 802.5 column (300 mm×8 mm) with THF at a flow rate of 1 ml/minwith a PEG calibration (range: 14,500 to 350 daltons) withrefractometric detection.

This removal of water carried out advantageously by simple evaporationcan be achieved in practice in a rotary evaporator or in a simple heatedreactor. However, the applicant has found that this procedure presents anumber of drawbacks.

In fact, the applicant has observed overheating in the reactor which cancause an increase in the high molecular weight oligomers. The applicantalso observed the presence of lactic acid in the water removed, hence aloss of raw material. These drawbacks are liable considerably to reducethe productivity of an industrial process and lead to a lack ofreproducibility.

SUMMARY OF THE INVENTION

The applicant has found a continuous method for obtaining an oligomericlactic acid composition rich in dimer and in trimer, from a lactic acidcomposition by removal of water from said lactic acid composition,characterized in that this removal is carried out in an adiabaticreactor fitted with a circulation loop equipped with heating means, inthat a lactic acid composition is continuously introduced into acirculating starter reaction medium, said reaction medium being heatedto a temperature between 120° C. and 180° C. and, preferably, between130° C. and 150° C., and in that the pure water is removed in vaporform.

According to the present invention, the oligomerization reactor is firstloaded with a predetermined quantity of an oligomeric composition ofstarter lactic acid obtained in particular by a batch process.

The volumetric quantity introduced at the start can be between 20% and80% of the reactor volume, and preferably, close to 50%.

According to the present invention, the circulation of the reactionmedium is advantageously provided by a centrifugal pump placed on thecirculation loop having an hourly delivery suitable for circulatingbetween 20 and 100 times the reaction mass.

BRIEF DESCRIPTION OF THE DRAWING

FIGURE 1 represents a device for practicing the method of the invention

DETAILED DESCRIPTION OF THE INVENTION

The reaction medium is heated to a temperature as previously defined bymeans of a heat exchanger placed on the circulation loop.

The removal of water according to the present invention is carried outat a pressure equal to or lower than atmospheric pressure. This isgenerally done at a pressure between 100 mbar and 200 mbar. The waterleaving the reactor in vapor form passes through an unchoking zone whichcan consist of a demister or of a distillation column.

According to the present invention, so-called “industrial”, “technical”,“food grade” or “pharmaceutical” grades of lactic acid compositions areused. Preferably, a lactic acid composition in water containing 88% byweight of lactic acid is used.

The oligomerization can be achieved in the presence of a homogeneousacidic catalyst such as H₂SO₄, H₃PO₄, methanesulfonic acid orparatoluenesulfonic acid, used in a weight quantity between 0.1% and 1%by weight of the quantity of dry lactic acid.

This method applies in particular to obtaining an oligomeric lactic acidcomposition comprising low molecular weight oligomers such as the dimer,trimer and tetramer. Preferably, the oligomeric lactic acid compositionaccording to the present invention has an MDP between 3 and 10.

The method according to the present invention offers the advantage ofobtaining linear oligomers with a narrow molecular weight distribution,without loss of raw material, that is, with a virtual absence of lacticacid in the recovered water.

This method can be put into practice by means of a device shownschematically in FIG. 1.

This device is characterized in that it comprises:

-   -   an adiabatic reactor (1) consisting of at least one distillation        column element equipped with heating mantles (hm), with a        circulation loop (2) equipped with a heat exchanger (3) and a        centrifugal pump (4);    -   a lactic acid composition feed (5).

The water leaving the reactor (1) in vapor form is introduced into anunchoking zone (6) from which it leaves at (7) and then passes throughthe heat exchanger (8) where it is condensed.

The oligomeric lactic acid composition leaves at (9).

The reactor is equipped with a level controller (10) by differentialpressure measurement.

According to the present invention, the lactic acid composition feed isimmersed in the reaction medium previously loaded into the reactor.

This method according to the invention offers great flexibility. Thecirculation rate of the reaction medium can be varied considerably,causing an excellent distribution of the heat within the reaction mediumand avoiding overheating, thereby obtaining a narrow molecular weightdistribution of the oligomers.

The unchoking zone serves to reduce the lactic acid evaporation rate,and serves to ensure the productivity of the industrial process.

The following examples illustrate the invention.

EXAMPLE 1 Apparatus

The oligomerization reactor (1) consists of two distillation columnelements (height: 0.8 meter, diameter: 180 mm) equipped with theirheating mantles (hm) to ensure the adiabaticity of the system.

The reaction medium is circulated by a centrifugal pump (4) delivering 1m³/h, said reaction medium passes through an electric heat exchanger (3)with a heat exchange area of 0.15 m² delivering a maximum heating powerof 6 kW.

The reactor is equipped with a level controller (10) by differentialpressure measurement with nitrogen injection.

In the following examples, the reactor is filled to 50% of its volume.

The removed water passes through a distillation column element (6) offour theoretical trays and is recovered after condensation in acondenser (8) with an area of 1 m².

Starting lactic acid:

It has a total lactic acid content—LAC—of 88.7 and a free water contentof 13.3%.

The LAC corresponds to the T in formula (1) mentioned above.

The mass of lactic acid monomer T contained in 100 g of oligomericlactic acid compositions is determined by the method described above.

Preparation of a “starter” batch then continuous tests according to theinvention:

About 25.8 kg of lactic acid are introduced and oligomerized in a batchfor 5 hours in the following conditions:

-   -   T° C exchanger=135° C.,    -   Pressure=100 mbar,    -   Power=2.5 kW.

After 5 hours, the reactor temperature is about 133° C., and theintroduction of 6.2 kg/h of the lactic acid composition into thereaction medium previously obtained is started. The pressure ismaintained at 100 mbar. The water is recovered at an average rate of 1.6kg/h. At the discharge of the centrifugal pump (4) the oligomer iswithdrawn at a rate of 4.6 kg/h. The LAC of the oligomer is measured at118%, corresponding to an MDP of 4.14 determined by the formula (1) inwhich T=118.

The production of oligomer was carried out for 50 hours.

EXAMPLE 2 (Not According to the Invention)

Carried out in an identical reactor to the one in example 1, but thelactic acid is introduced into the top of the reactor, and the waterremoval circuit is direct, that is, without the column (6) of fourtheoretical trays. The operation is identical. When the reactortemperature reaches 133° C., the lactic acid is introduced at a rate of6.26 kg/h. The water is recovered at a rate of 1.88 kg/h, and theoligomer at a rate of 4.38 kg/h containing 115% of LAC. The aqueousstream is found to contain about 18% of entrained lactic acid.

EXAMPLE 3 (Not According to the Invention)

About 31 kg of lactic acid with 88.7% of LAC and 13.3% of water areintroduced into a stirred reactor with a volume of 40 l heated by meansof a double jacket and able to operate under vacuum. The mixture isheated with stirring, gradually reducing the pressure to 100 mbar. Toreach the temperature of 135° C. in the reactor, the heat transfer fluidmust be heated to 155° C. After 5 hours of reaction, the lactic acid isintroduced into the top of the reactor at a rate of 6.3 kg/h. The wateris recovered at a rate of 1.8 kg/h. An oligomer is obtained at a rate of4.3 kg/h containing 113% of LAC.

1. A continuous method for obtaining an oligomeric lactic acidcomposition from a lactic acid composition by removal of water from saidlactic acid composition, characterized in that this removal is carriedout in an adiabatic reactor fitted with a circulation loop equipped withheating means, in that a lactic acid composition is continuouslyintroduced into a circulating starter reaction medium, said reactionmedium being heated to a temperature between 120° C. and 180° C., and inthat the pure water is removed in vapor form.
 2. The method as claimedin claim 1, characterized in that the circulating reaction medium isheated to a temperature between 130° C. and 150° C.
 3. The method asclaimed in claim 1, characterized in that the volumetric quantity ofstarter composition introduced into the reactor at the start is between20% and 80% of the reactor volume.
 4. The method as claimed in claim 3,characterized in that the volumetric quantity of starter compositionintroduced into the reactor at the start is close to 50% of the reactorvolume.
 5. The method as claimed in claim 1, characterized in that thereaction medium circulates in the apparatus consisting of the adiabaticreactor and its circulation loop between 20 times and 100 times perhour.
 6. The method as claimed in claim 1, characterized in that thewater is removed at a pressure equal to or lower than atmosphericpressure.
 7. The method as claimed in claim 6, characterized in that thewater is removed at a pressure between 100 mbar and 200 mbar.
 8. Adevice suitable for putting the method into practice according to anyone of claims 1 to 10, said device being characterized in that itcomprises: an adiabatic reactor (1) consisting of at least onedistillation column element equipped with heating mantles (hm), with acirculation loop (2) equipped with a heat exchanger (3) and with acentrifugal pump (4); a lactic acid composition feed (5); an unchokingzone (6); a water vapor discharger (7) fitted with a condenser (8); adischarge of the oligomeric lactic acid composition (9); a levelcontroller (10).
 9. The device as claimed in claim 8, characterized inthat the lactic acid composition feed is immersed in the reaction mediumpreviously loaded into the reactor (1).
 10. The device as claimed inclaim 8, characterized in that the unchoking zone is a distillationcolumn.